Fiber reinforced plastics (hereinafter abbreviated to "FRPs") are well-known fiber reinforced materials. Various fibers including glass fibers and carbon fibers have been developed for use in FRPs. These fibers have a strength equivalent to or greater than those of steels. Thus, FRP structural members are capable of replacing reliable steel structural members and various FRPs have been applied to structural members for supporting loads.
On the other hand, since the specific gravity of FRPs, in general, is on the order of one-fifth the specific gravity of steels, the application of FRP structural members to construction structures effectively reduces the total weight of the structures.
FIG. 1 shows a beam member, an exemplary structural member to which the present invention pertains, comprising an elongate beam body 1 formed of a FRP, bearing pads 2 and 3 attached to the opposite ends of the beam body 1, respectively, and a load bearing part 4 provided at the middle of the beam body 1 for supporting an external load. When the beam body 1 supported at the opposite ends thereof is loaded in the middle portion thereof, the bending moment is large in the middle portion and small at the end portions. Since the elastic modulus of such a fiber reinforced plastic structural member is 1/20 to 1/5 that of steels, ordinarily, the beam body 1, namely, the structural member, is provided with an internal reinforcement so as to tend to uniformly distribute the bending stress throughout the section thereof.
FIG. 7 illustrates an exemplary conventional structural member provided with an internal reinforcement. This structural member comprises a beam body 1 having a rectangular cross section, a reinforcing plate 5 formed of the same material as the beam body 1 and provided inside the beam member 1, and a filling material 6 such as, for example, foam polyurethane, filling the inner space of the beam body 1. In this structural member, however, since the reinforcing plate 5 is joined along the narrow side edges thereof to the inner surface of the beam body 1, the strength of the junctions is not large enough to withstand tensile forces and shearing forces which act on the junctions, and hence such a conventional structural member is not satisfactory in reliability.